Yes, it is possible to handle forms with Shortcode Exec PHP. I have added a basic example here on the support page. Error 501 is a rare and strange error. Maybe there is a problem with the function add_email_to_list?

Error 501 means something like not implemented. A lot of people are using the plugin successful, so maybe there is a server feature missing or disabled preventing the plugin from doing its work. However, the plugin does not have important requirements (not more than WordPress itself), maybe only that the PHP eval function can be executed. If you like, I can take a look. Use the contact form (http://blog.bokhorst.biz/contact/) to sent me your login details (preferably FTP) if you want to. Btw, you get faster response if you use the support page of the plugin (http://wordpress.org/extend/plugins/shortcode-exec-php/).